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Food 101

The Acid Test

By Robert L. Wolke

Wednesday, November 10, 2004; Page F01

Your column on raw fish and sushi reminded me of something I've always wondered about: seviche, the Latin American seafood dish. Books say the fish is "cooked" just by marinating it in lime juice. Is it really "cooked," or is it still raw?

Those quotation marks around "cooked" have been driving me nuts for years. Virtually every mention of ceviche (seh-VEE-chay; I'll use the Spanish spelling) by food writers is accompanied by a gratuitous statement to the effect that lime juice does to protein what heat does to protein, and therefore that the fish is essentially "cooked" by the lime juice.

But before I serve up your mini-course in protein chemistry, here's a bit of an appetizer.

Ceviche is made from small pieces of any of several kinds of raw saltwater fish, or from scallops or other shellfish, or squid or octopus, all marinated in lime juice for several hours in the refrigerator, after which some oil, usually chopped vegetables and sometimes spices are added before the dish is served cold. If the fish is fresh to begin with -- and it absolutely must be -- it is safe to marinate it for up to five or six hours, because the lime's acidity is more than strong enough to prevent bacterial growth.

But is it cooked?

The citric acid in lime juice changes the proteins in fish by a process called denaturation. The normally twisted and folded protein molecules are unraveled or unfolded into less convoluted shapes, and the shapes of molecules, especially proteins, are responsible for most of their physical and chemical properties. In other words, they have lost their original natures: They have been denatured.

And yes, cooking also denatures proteins.

But besides acids and heat, a variety of other kinds of situations can denature proteins. High concentrations of salts, including table salt (sodium chloride) can do it. Air can do it, as happens in the bubbles formed when cream is whipped. Even alkalis, the opposite of acids, and low temperatures, the opposite of heat, can do it, but less commonly. The cooking analogy comes only from the fact that heat is the most familiar protein-denaturing agent in the kitchen.

Denaturing or unwinding protein molecules is no great trick, because the bonds that keep them twisted and folded aren't very strong. Evolution may supply a rationale for that fact: Over the eons, specific proteins have evolved to do specific jobs in specific living organisms, so they have no need to be stable under conditions vastly different from those that prevail in the organisms they serve. Thus, meat and fish proteins can be destabilized when subjected to higher acidities and higher temperatures than those in the animal's muscles. Animal muscle is normally only slightly acidic, while body temperatures are relatively low, especially in the case of sea creatures. That's why in making ceviche, fish protein can be denatured by an acid no stronger than lime juice, and even at refrigerator temperatures.

The different denaturing methods complement and enhance one another. For example, the stronger the acid that a protein is subjected to, the lower the temperature at which it will be denatured by heat. That's why meat or fish bathed in a marinade containing lemon or lime juice (citric acid), vinegar (acetic acid) or wine (primarily tartaric and malic acids) will require less cooking time than an unmarinated sample. And if you want to explain that by saying the acid has partially "cooked" the meat, I can't stop you.

The Nature of Denaturing

After the protein molecules in a food have been unraveled or unfolded by any of these denaturing environments, they may not stay that way. For one thing, if the conditions should change, they can re-ravel back into their original shapes or something similar. But usually this doesn't happen, because as they unfold or disrobe, so to speak, the protein molecules expose sections of themselves that had previously been concealed in the folds, and these sections can react with other chemicals in the environment that change their shapes more or less permanently.

Or the newly denuded sections can bond to one another, making so-called cross-links that knit the molecules together into tighter structures. That's why when you either cook a piece of fish or soak it in lime juice to make ceviche, it develops a firmer texture. You'll notice also that it becomes more opaque, because light rays can't penetrate the tightly balled-up, cross-linked protein molecules. (The same thing happens to the protein in egg white; when cooked it turns from transparent to opaque white.)

And under the right conditions, acidified, unfolded protein molecules will stick together and the protein will coagulate, as when cheese curds are formed when lactic acid denatures the casein in milk.